1. Field of the Invention
The invention relates to a vehicle control device that executes electronic control in cooperation with a power transfer mechanism.
2. Description of the Related Art
Generally, a road surface changes more frequently when a vehicle drives on a bad road (hereinafter, referred to as “off road”) than when the vehicle drives on a paved road (hereinafter, referred to as “on road”), so large variations occur in the driving condition of the vehicle.
For example, when the vehicle drives on a snow-covered road surface having a small friction coefficient among off roads, an appropriate torque cannot be obtained when a torque is generated in accordance with a throttle characteristic similar to that of the on road to cause a slip, or the like, to occur. Thus, it is difficult to obtain desired driving capability.
In addition, in a case where a four-wheel drive vehicle runs on a rugged mountain road, for example, when a left rear wheel is not in contact with a ground and the other three drive wheels are in contact with the ground, the rotational resistance of the left rear wheel steeply decreases. Therefore, owing to the function of a differential mechanism, which serves as a differential unit, provided for the vehicle, the power of an engine is intensively transferred to the left rear wheel that is not in contact with the ground, so the rotational speed of the left rear wheel remarkably increases, and it becomes difficult for the power to be transferred to the other three drive wheels that are in contact with the ground.
In this way, when the left rear wheel, of which the rotational speed has remarkably increased, contacts the ground again, because the power of the left rear wheel is larger than the power of each of the right rear wheel and front wheels, so the traveling direction of the vehicle deviates rightward. As such a phenomenon alternately and successively occurs in the left and right wheels, fishtail occurs. Therefore, it is difficult to obtain desired driving capability.
Thus, as described above, variations in the vehicle driving condition due to a change of a road surface are more frequent when the vehicle drives on an off road than when the vehicle drives on an on road, so it is necessary to control the vehicle differently when the vehicle drives on an off road from when the vehicle drives on an on road.
Then, there has been suggested a vehicle control device that maintains good driving capability while driving on various types of off roads. The above vehicle control device defines the slip ratio of each drive wheel by a difference between the rotational speed of each drive wheel and the traveling speed of the vehicle, and, when the slip ratio of any of the drive wheels is higher than a predetermined value, applies braking force to that drive wheel. Thus, a drive wheel that is not in contact with a ground surface and has a high slip ratio is braked to suppress idle rotation, so owing to the function of a differential mechanism, power is desirably transferred to the drive wheels that are in contact with the ground surface to improve off road driving capability (for example, see Japanese Patent Application Publication No. 2004-90886 (JP-A-2004-90886)).
In addition, for a power control mechanism, such as an electronically controlled throttle and an automatic transmission, there has been suggested a technique that is able to change a driving mode in accordance with a road surface. For example, there is a technique that, when the vehicle drives on a snow-covered road surface, the rate of change of the opening degree of the electronically controlled throttle with respect to the operation amount of an accelerator pedal is decreased as compared with that when the vehicle drives on an on road.
In this way, in some of the existing four-wheel drive vehicles, an ECU controls shifting of a power control mechanism, such as adjustment of the opening degree of a throttle valve and shifting of the speed ratio of a transmission. In some of the above four-wheel drive vehicles, a driver manually operates to mechanically perform shifting of a power transfer mechanism, such as shifting between the lock and free of a differential mechanism and shifting between the high-speed gear and low-speed gear of a transfer that serves as a power transfer device.
However, in order to improve off-road driving capability, the power transfer mechanism and the power control mechanism need to be integrally controlled in accordance with a road surface condition. If the above mechanisms are separately controlled, it is difficult to obtain desired driving capability, causing a driver to feel discomfort.
Then, in order to integrally control the power transfer mechanism and the power control mechanism in accordance with a road surface condition, there is a conceivable manner that the ECU collectively controls the power transfer mechanism and the power control mechanism.
However, in the existing four-wheel drive vehicles, when it is attempted to control the power transfer mechanism and the power control mechanism by the ECU, it requires a big design change in order to allow the ECU to perform shifting of the differential mechanism and shifting of the transfer, which have been mechanically performed so far. This problematically increases additional cost.
In addition, in order to retrofit the above function to the existing four-wheel drive vehicle at low cost without a big design change, it is desirable to provide a vehicle control device that shifts a driving mode in such a manner that, in accordance with a driving mode input by the driver, the ECU shifts the power control mechanism and the driver is instructed to mechanically shift the power transfer mechanism. However, when the power control mechanism is shifted in accordance with the intended driving mode before completion of shifting of the power transfer mechanism, and when the driving mode returns to the original one because shifting of the power transfer mechanism has not been completed within a certain period of time, the power control mechanism needs to be shifted to the original state again. This problematically causes an unnecessary shift of control and, as a result, deteriorates drivability.